In the chemical industry brick and/or ceramic-lined vessels are generally utilized to provide a means in which materials are mixed, and chemical reactions or other processes are carried out, to produce intermediate and final chemical products. Such a vessel, used for example to contain reagents undergoing a chemical reaction,generally comprises a metal casing with an internal ceramic lining. The vessels are of differing sizes and shapes, depending on specific operational parameters, but typically are cylindrical and of the order of three feet or greater in diameter and ten feet or more in length. The vessels are specially designed to incorporate apertures for ingress of the reagents and egress of the final reaction products. The outer metal casing is usually steel, nickel or an alloy thereof, or other suitable material selected for both durability and corrosion resistance. The internal lining is usually glass or other ceramic material (example; silica, magnesia or alumina, or similar heat and chemical resistant material). In large size reactors the ceramic lining is frequently fabricated of bricks, chosen for thermal insulation and for resistance to attack by the chemical species present in the vessel. An inner lining of ceramic layer, which contacts the reaction mixture usually has the same composition as the brick material, or may be a different composition chosen primarily on the basis of its compatibility with the reaction mixture. For example, the inner lining may be carbon or carbon bricks for improved resistance to chemical attack, wherein thermal insulation is furnished mainly by the interlayer of ceramic bricks between the carbon-loaded lining of the bricks and the outer metal wall. Typically in these designs at least one access port is provided in the vessel wall for maintenance purposes.
Reaction vessels similar to those described above are generally used to contain and control industrial scale chemical reactions. Frequently these reactions must be carried out at elevated temperatures to obtain a good yield for the desired product. Since the reaction vessel has considerable mass, it must be preheated to its operating temperature over a period of hours before admitting the reagents and initiating the desired reaction. If the reaction is interrupted or temporarily shut down and the reactor cools, the preheating process must often be repeated before the reaction can be restarted in a satisfactory manner. This heating process often generates combustion products and effluents which are environmentally undesirable.
An example of such a reaction is an organic chemical feedstock operating at high temperature to carry out the synthesis of a value added product. These reactions sometimes involve particularly severe conditions, and have a high likelihood of unwanted side products, thereby creating a disposal problem if the reaction conditions are not well controlled. In this context the reactor temperature is an extremely important process variable.
In many cases conventional heating by, for example, the burning of a mixture of natural gas and air within the reaction vessel, is not practical because the interior of the reaction vessel must be kept free from water, one of the by-products of the combustion of natural gas. In addition, if the reactor vessel happens to contain a combustible lining, for example carbon bricks, air or oxygen based combustion within the reactor is likely to erode the lining by oxidation to CO and CO.sub.2.
One prior art method is to use the production reaction itself as a means to preheat the vessel to its correct operating temperature. However, because of the relatively expensive organic feedstock which is consumed under the non-optimum conditions, this procedure has several disadvantages, as well as being cost prohibitive. Additionally, undesired side reactions may form side products during such a heating process, while the reactor is coming up to its final temperature but is still cooler than the optimum temperature. These produced side products, if formed, must be disposed of at a considerable cost in addition to the expense incurred as a result of the feedstock consumed.
Another prior art method is the chlorinating of an inexpensive hydrocarbon, for example methane. This reaction forms carbon tetrachloride and hydrogen chloride which in the past were two compounds that were classified as low value but saleable products. However, in view of the regulatory situation that has changed, carbon tetrachloride itself has been listed as a Class II substance under the 1989 Montreal Protocol and will undergo stepwise phase out to 85 percent of non-CFC feedstock use by Jan. 1, 1995 and total phase out by Jan. 1, 2000. Although carbon tetrachloride produced in the above manner may be exempt under various regulatory provisions, the regulatory situation may become more stringent. Therefore the elimination of a process dependent on carbon tetrachloride reaction is highly desirable, and use of microwave heating as a substitute heating means would constitute a significant improvement.
It is therefore an object of the present invention to provide a means of utilizing electrically generated microwave energy for the heating of reaction vessels, which avoids the formation of environmentally undesirable compounds, as well as the consumption of expensive chemical feedstock used as a heating fuel.
It is a feature of the present invention to provide a uniform distribution of microwave energy, in a whispering gallery mode of energy propagation, throughout the vessel, thereby avoiding the formation of local regions of high field intensity and hot spot formation resulting therefrom.
It is a further feature of the present invention to provide an applicator window which causes minimal reflection or absorption of microwave energy, while still maintaining its structural integrity while exposed to heat transfer and chemical attack from the species present in the chemical vessel.